I. Field of the Invention:
This invention relates generally to an electronic medical instrumentation system and more specifically to a real time data processing and display system whereby respiratory system impairments can be determined.
II. Discussion of the Prior Art:
In a paper entitled "Abnormal Regulation of Ventilation in Infants at Risk for Sudden-Infant-Death Syndrome" by Daniel C. Shannon et al, published in the Oct. 6, 1977 issue of The New England Journal of Medicine, Volume 297, No. 14 at pp 747-50, there is described a method for determining the sensitivity of the infant's respiratory response to variations in concentrations of inspired CO.sub.2. In normal infants, an increase in inspired CO.sub.2 results in an increase in the ventilation. However, with infants who are subject to chronic sleep hypoventilation or periods of prolonged apnea during sleep, it was found that there was a significant reduction in the change in ventilation per change in partial pressure of carbon dioxide in the inspired breath. Those infants that had a history of prolonged sleep apnea episodes exhibited a change in minute ventilation in response to carbon dioxide breathing which was significantly less than that in the control group of infants. This difference was attributed primarily to a significantly smaller increase in tidal volume, which accounts for most of the ventilatory response in normal infants. Dr. Shannon and his co-workers concluded, based upon studies conducted, that impaired regulation of alveolar ventilation is an explanation for sleep apnea.
The ability of the respiratory system to react to changes in gas concentration can be measured and the sensitivity of the respiratory system noted. Results of such tests have produced useful information, and demonstrated variances in sensitivity according to the maturity of the respiratory system. The ventilatory response to decreased levels of oxygen in adults is far less than that measured in neonates. Neonates will show a measurable increase in breathing rate when they inspire a mixture of 18% O.sub.2 rather than the normal 21%. The normal respiratory response to inspired CO.sub.2 is an increase in ventilation for an increase in alveolar or arterial CO.sub.2 concentration. The slope of the derived curve of ventilation versus CO.sub.2 concentration may then be used as an index of how sensitive the respiratory system is to changes in CO.sub.2 concentration.
Ventilatory response measurements, when taken on pre-term infants, tend to show a flatter slope, ##EQU1## where V.sub.E initial is the ventilation at normal inspired CO.sub.2 levels and V.sub.E final is the ventilation at an increased CO.sub.2 level. PACO.sub.2 initial and PACO.sub.2 final refer to the partial pressure of the alveolar or end tidal CO.sub.2 at normal and increased inspired CO.sub.2 levels, respectively. The decrease has been shown to be directly related to the maturity of the central nervous system (CNS). Hence, by measuring the slope of normal ventilation versus levels of CO.sub.2 while maintaining the inspired oxygen concentration constant at 21%, one is able to determine the relative maturity of the respiratory system. Where a lower than normal slope is exhibited, a physician will be better able to inform the parents as to the risk of SIDS. With such information, the parents may be relieved of unnecessary worry or, alternatively, they will be instructed such that they can have the infant monitored and medically treated during the immature state. The respiratory maturity typically occurs within the first few months but may not occur for a period of 24 months in a small population of infants at risk.
Systems to measure metabolic rate or physiological changes due to exercise stress are somewhat related to the present invention. A system of this type is described in the Rummel et al U.S. Pat. No. 3,799,149 and includes apparatus for measuring CO.sub.2 and O.sub.2 in inspired and expired breath as well as changes in ventilation. The apparatus described in the Rummel et al patent, while perhaps satisfactory for adult subjects, is totally unsatisfactory for use with infants. For the system of the Rummel et al patent to operate satisfactory, the sample size necessary for measurement of the gases is too large to be used with infants and drastically affects the ventilation as well as gas concentration data. Because infants are more sensitive to the introduction of sampling devices into the mouth or nasal passages than are adults, it is difficult to obtain accurate data when it is noted that consistent breathing patterns tend only to be available when the infant is in a "deep sleep" state as distinguished from REM (rapid eye movement) sleep. Unless a deep sleep condition can be reached, the data obtained from systems of the prior art are subject to error. Anything which interferes with the ability to achieve the deep sleep state, such as the presence of an uncomfortable object in the patient's nose, is therefore to be avoided.